This invention is generally related to magnetic resonance imaging (MRI) using nuclear magnetic resonance (NMR) phenomena. It is particularly directed to a method and corresponding apparatus for more efficiently capturing and providing MRI data suitable for use in multi-dimensional imaging processes.
MRI is by now a widely accepted, medically important and commercially viable technique for obtaining digitized visual images representative of internal body tissue and structures. There are many commercially available approaches and there have been numerous publications describing these and other approaches to MRI. Many of these use multi-dimensional Fourier transformation techniques which are now well-known to those skilled in this art.
In general, MRI devices establish a constant homogeneous magnetic field to orient nuclear spins, apply a specific additional bias field gradient in a known plane or region under consideration, and apply a radiofrequency pulse or a sequence of pulses to perturb the nuclei. These nuclei in the known bias field gradient emit an RF signal in a specific band determined by the magnetic field distribution, and these RF emissions are detected by receiving coils and stored as a line of information in a data matrix known as the k-space matrix. The full matrix is built up by successive cycles of conditioning the spins, perturbing them, and collecting RF emissions. An image is then generated from this matrix by Fourier transformation, which converts the frequency information present in the RF oscillations to spatial information representing the distribution of nuclear spins in tissue or other imaged material.
Magnetic resonance imaging has proven to be a valuable clinical diagnostic tool in a wide range of organ systems and pathophysiologic processes. Both anatomic and functional information can be gleaned from the MR data, and new applications continue to develop with each improvement in basic imaging technique and technology. As technologic advances have improved achievable spatial resolution, for example, increasingly finer anatomic details have been able to be imaged and evaluated using MR. At the same time, fast imaging sequences have reduced imaging times to such an extent that many moving structures can now be visualized without significant motion artifacts.
Often, however, there is a tradeoff between spatial resolution and imaging time, since higher resolution images require a longer acquisition time. This balance between spatial and temporal resolution is particularly important in cardiac MR, where fine details of coronary artery anatomy, for example, must be discerned on the surface of a rapidly beating heart. A high-resolution image acquired over a large fraction of the cardiac cycle will be blurred and distorted by bulk cardiac motion, whereas a very fast image may not have the resolution necessary to trace the course and patency of coronary arteries. Some of the fastest imaging sequences currently implemented, such as echo planar imaging (EPI), approach the goal of yielding images of reasonable resolution in a suitably short fraction of the cardiac cycle. Other approaches have also been tried to eliminate the effects of cardiac motion, including k-space segmentation, in which image acquisition is divided up over several cardiac cycles with ECG gating to ensure that the heart is in the same phase of systole or diastole during acquisition of each segment. Cine images of multiple cardiac phases may be pieced together with this technique, with partial acquisitions for different phases occurring in each cardiac cycle. One problem with this class of techniques is that respiratory motion can change the position of the heart over the course of several cardiac cycles. Partial acquisitions will then be misregistered, and artifacts will result. In an attempt to eliminate or adjust for respiratory motion, breath holds, respiratory gating, and navigator echo gating techniques have all been tried, and each of these techniques has had some signficant successes. Nevertheless, an imaging strategy which allowed high-resolution images to be acquired comfortably within one or two phases of the cardiac cycle would circumvent many of the difficulties and residual artifacts associated with these compensation techniques.
The speed with which magnetic resonance (MR) images may be acquired has increased dramatically over the past decade. The improvements in speed may be traced to a combination of advances in the technologies of magnet construction and actuation, and innovations in imaging strategy. Strong, fast-switching magnetic field gradients and fast electronics have allowed the intervals between data collections to be reduced significantly. Meanwhile, fast gradient-echo and spin-echo sequences have reduced image acquisition time by allowing greater portions of k-space to be sampled after each spin excitation. Echo planar imaging (EPI), fast low-angle shot (FLASH), turbo spin echo (TSE), and spiral imaging techniques all allow very short intervals between acquisition of successive data points. The DUFIS, OUFIS, RUFIS, and BURST family of sequences further reduce image acquisition time by eliminating time delays incurred during gradient switching and echo formation. Details of the above-mentioned eight techniques may be found in the following papers: P. Mansfield, Multi-planar image formation using NMR spin echoes. J. Phys. C. 10, L55-58 (1977); A. Eaase, J. Frahm, D. Mattaei, W. Hanicke, K. D. Merboldt, FLASH imaging: rapid NMR imaging using low flip-angle pulses. J. Magn. Reson. 67, 256-266 (1986); J. L. Listerud, S. Einstein, E. Outwater, H. Y. Kressel, First principles of fast spin echo. Magn. Reson. Q. 8, 199-244 (1992); C. Meyer, B. B. Hu, D. Nishimura, A. Macovski, Fast spiral coronary artery imaging. Magn. Reson. Med. 28, 202-213 (1992); I. J. Lowe, R. E. Wysong, DANTE ultrafast imaging sequence (DUlFIS). J. Magn. Reson. Ser. B 101, 106-109 (1993); L. Zha, I. J. Lowe, Optimized ultra-fast imaging sequence (OUFIS). Magn. Reson. Med. 33, 377-395 (1995); D. P. Madio, I. J. Lowe, Ultra-fast imaging using low flip angles and FIDs. Magn. Reson. Med. 34, 525-529 (1995); and J. Hennig, M. Hodapp, Burst imaging. MAGMA 1, 39-48 (1993).
Increasing the speed of MR imaging further is a challenging proposition, since the aforementioned fast imaging techniques have already achieved an impressive efficiency. All these techniques allow very short intervals between acquisition of successive data points, and hence do not waste much time in accumulating the data for the k-space matrix required to generate an image. In flow-encoded EPI images, for example, the entire complex k-space matrix is filled in a single spin excitation (which is followed by multiple spin conditioning cycles involvinig the application of multiple stepped field gradients), and the resulting image matrix is likewise "full," with useful information stored in both the real and the imaginary channels. One common feature of nearly all the fast imaging techniques currently in use, however, is that they all acquire data in a sequential fashion. Whether the required data set, i.e., the k-space data matrix, is filled in a rectangular raster pattern, a spiral pattern, a rapid series of line scans, or some other novel order, it is acquired one point and one line at a time.
That is, the prior art in fast MR imaging has concentrated on increasing the speed of sequential acquisition by reducing the intervals between scanned lines. Modifications to pulse sequences or to magnetic field gradients have yielded a gradual improvement in imaging speed by allowing faster sequential scanning of k-space, but these improvements face limits due to the intervals necessary to create, switch or measure the magnetic fields or signals involved in data acquisition. It would therefore appear difficult to devise a sequential technique with significantly better efficiency than the current fast imaging techniques.
Several fast imaging schemes have been proposed to date using simultaneous data acquisition in multiple RF coils, as described in: D. Kwiat, S. Einav, G. Navon, A decoupled coil detector array for fast image acquisition in magnetic resonance imaging. Med Phys, 18:251-265 (1991); D. Kwiat, S. Einav, Preliminary experimental evaluation of an inverse source imaging procedure using a decoupled coil detector array in magnetic resonance imaging. Med Eng Phys, 27, 257-263 (1995); J. W. Carlson, T. Minemura, Imaging time reduction through multiple receiver coil data acquisition and image reconstruction. Magn Reson Med 29, 681-688 (1993) and U.S. Pat. No. 4,857,846 of J. W. Carlson; and J. B. Ra, C. Y. Rim, Fast imaging using subencoding data sets from multiple detectors. Magn Reson Med 30, 142-145 (1993). These approaches have offered the promise of significant savings in image acquisition times.
The Carlson and Minemura paper describes a twofold acquisition time savings using two nested body coils. In their approach, partial data sets are collected simultaneously in the two coils, one of homogeneous sensitivity and the other with a linear gradient in sensitivity. Missing lines in k-space are generated using a series expansion in terms of other phase-encoded lines. This approach using body coils appears to require that a significant portion of the data for the partial k-space matrix be acquired before any of the missing lines can be filled in by postprocessing, and thus does not allow for the missing lines to be built up as the data arrives, in real time. The approach uses coil sensitivity information in place of some portion of the gradient phase encoding steps, but has drawbacks. The coils used by Carlson and Minemura are body coils, which provide large volume coverage but lower overall sensitivity than surface coils, and it would be difficult to augment their number to improve time savings.
The approach of Ra and Rim involves a simultaneous acquisition technique in which images of reduced FOV are acquired in multiple coils of an array and the Nyquist aliasing in those images is undone by reference to component coil sensitivity information. The unaliasing procedure involves a pixel-by-pixel matrix inversion to regenerate the full FOV from multiple copies of the aliased image data. The "subencoding" technique of Ra and Rim relies on estimates of component coil sensitivities by effectively probing the sensitivity at each pixel. This pixel-by-pixel approach can lead to local artifacts; for example, the matrix inversion can begin to fail in regions of low sensitivity. Further, by its very nature as a pixel by pixel dealiasing approach, the Ra & Rim method is computation-intensive and is limited to postprocessing, as all image data must be present before the reconstruction can be undertaken.
In a related area, multiple coil signal collection has been used in MR phased array systems as reported in R. B. Roemer, W. A. Edelstein, C. E. Hayes, S. P. Souza, and O. M. Mueller, The NMR phased array. Magn. Reson. Med. 26, 192-225 (1990); C. E. Hayes and P. B. Roemer, Noise correlations in data simultaneously acquired from multiple surface coil arrays. Magn. Reson. Med. 16, 181-191 (1990); C. E. Hayes N. Hattes, and P. B. Roemer, Volume imaging with MR phased arrays. Magn. Reson. Med. 18, 309-319 (1991). The increased information content of the multiple received signals in such systems has been used to increase the signal-to-noise ratio (SNR) of MR images. Since their initial description, phased arrays have seen increasing use in clinical MR imaging. For example, the improvements in SNR provided by phased arrays have allowed significant advances in imaging of the pulmonary vasculature as reported by T. K. F. Foo, J. R. MacFall, C. E. Hayes, H. D. Sostman, and B. E. Slayman, Pulmonary vasculature: single breath-hold MR imaging with phased array coils. Radiology 183, 473-477 (1992). Still, with a few notable exceptions, the bulk of phased array applications have addressed increased sensitivity, with little effort towards improving image acquisition speed or spatial resolution.